1. Field of the Invention
This invention relates to an image forming apparatus using an electrophotographic process. More particularly, this invention relates to improvement in and concerning a method for the supply of toner to a developing device.
2. Description of Related Art
An image forming apparatus such as a copying machine or a printer which uses the electrophotographic process incorporates therein a developing device adapted to supply a toner from the toner replenishing tank containing the toner for the purpose of developing an electrostatic latent image formed on the surface of a photosensitive medium.
Particularly, the image forming apparatus of the type designed to use a two-component developer consisting of a toner and a carrier is required to keep the ratio of the toner to the carrier (toner concentration) in the developer within a stated proper range. The developing device, therefore, is adapted to supply a prescribed amount of the toner from the toner replenishing tank whenever the toner concentration in the developer falls below a prescribed level.
U.S. Pat. No. 4,592,645 discloses one example of what may be termed a toner concentration controlling device, which is adapted to discern the toner concentration in the developer by detecting the permeability of the developer by means of a sensor disposed in the developing device and comparing the detected value of the permeability with a reference value.
Another device of the so-called automatic image density control (AIDC) method which discerns the toner concentration in the developer by forming an electrostatic latent image of a reference pattern on a photosensitive medium, developing this latent image, and detecting the image density of the developed image with a photosensor and effects the supply of toner whenever the detected toner concentration falls below a prescribed level has been widely know.
FIG. 19 is a cross-sectional model diagram illustrating a typical developing device using a two-component developer and FIG. 20 is a model diagram illustrating the movement of the developer in the developing device and the consumption of the toner. In these diagrams is depicted the developing device which effects the development of a latent image by the magnetic brush developing method. This developing device has a first screw vane 16, a bucket roller 11, and a developing sleeve 6 incorporated in a casing 3 sequentially in the order mentioned in the direction toward a photosensitive drum 1. These component parts are adapted to be driven jointly by one motor (not shown). The first screw vane 16 is possessed of a multiplicity of semicircular spiral parts and is rotated counterclockwise. The bucket roller 11 is possessed of a multiplicity of L-shaped vanes extending in the radial direction and a second screw vane 13 and is rotated clockwise. The developing sleeve 6 is located outside a magnet roller 7 fixed and not allowed to rotate and is rotated clockwise. In the casing 3, a partition board 5 is disposed between the first screw vane 16 and the bucket roller 11. At the opposite terminals of this partition board 5, notches 5a, 5b are formed as illustrated in FIG. 20.
In the developing device described above, the toner from the toner supply part (not shown) is supplied to above one terminal part of the first screw vane 16. By the bucket roller 11, the developer which has been already used for development is passed through the notch 5a formed at one terminal of the partition board 5 and returned to the screw vane 16. It is then stirred in conjunction with the replenishing toner and, at the same time, conveyed in the direction of the arrow by the screw vane 16. At this time, the toner in transit is charged to a prescribed polarity owing to the friction with carrier particles.
The developer thus conveyed is stirred and is supplied into the bucket roller 11 through the notch 5b formed at the other terminal of the partition board 5. Here, the developer is stirred by the vanes 14 owing to the rotation of the bucket roller 11 and is moved by the magnetism of the magnet roller 7 in the direction of the outer periphery of the sleeve roller 6. The toner is consumed by being deposited by means of the sleeve roller 6 on the electrostatic latent image formed on the outer peripheral surface of the photosensitive drum 1. The developer as accompanied by the consumption of the toner is conveyed to the lefthand end part in the view of FIG. 20 by the second screw vane 13 disposed inside the bucket roller 11 and then supplied through the notch 5a onto the first screw vane 16. In this manner and as illustrated in FIG. 20, the developer is circularly moved along a conveying path formed jointly by the first screw vane 16, the second screw vane 11, and the notches 5a, 5b. During the course of the conveyance of the developer, the toner is consumed and replenished.
In the developing device described above, since the replenishment of the toner is effected whenever the fall of the toner concentration below the prescribed reference value is detected, this toner replenishment is fated to be repeated and the toner is supplied in a large amount at once where the toner is consumed quickly and copiously as in the continuous copying of an image on a large number of copying papers. As a result, the toner is not sufficiently dispersed in the developer and some of the toner escapes being fully charged by friction with the carrier and remains to be a poorly charged toner. The occurrence of this poorly charged toner entails the phenomenon of feeble attachment of toner to the white background of a copying paper (hereinafter referred to as "toner fog") and the problem that the toner is scattered inside the copying device to defile the interior of the device and smear the copying paper.
If the amount of the toner to be supplied at a time is set at a relatively small level for the purpose of precluding the problem mentioned above, then the supply of toner cannot keep abreast with the consumption of toner and the toner concentration in the developer decreases and the density of the developed image proportionately dwindles. There consequently arises the problem that the carrier is suffered to adhere to the photosensitive drum.
Where a subject copy to be copied under the condition of very quick consumption of toner happens to contain an image whose density is varied very widely from one portion to another, the amount of the toner to be consumed heavily varies locally on the copying paper and consequently the toner concentration is varied inside the developing device between the portion having a large toner consumption and the portion having a small toner consumption, possibly with the result that the developed image betrays uneven distribution of density.
In the developing device described above which causes the toner to be conveyed in one direction from the part in which the toner has been received, since the toner is gradually consumed as the developer is moved from one end to the other of the developing device by the second screw vane 13 as illustrated in FIG. 19, inherently the toner concentration is higher in the righthand part than in the lefthand part of the bucket roller 11 in the bearings of FIG. 20. In FIG. 20, for example, even if the righthand end part of the photosensitive drum 1 has a proper image density, the lefthand end part thereof constitutes itself a region for a low image density. Under the condition involving quick consumption of the toner, therefore, the concentration of the toner attached to the outer peripheral surface of the developing sleeve 6 varies widely between the opposite end parts and, as a result, a inclination occurs in the density of the developed image.
The toner concentration inside the developing device is not evenly increased promptly in response to the replenishment of toner. In the developing device of this kind, therefore, it has been customary for the toner to be replenished in a prescribed amount at prescribed intervals. To be specific, after the toner is supplied by way of replenishment in the prescribed amount, the developer is stirred and conveyed for a prescribed time and then the toner replenishment is repeated based on the detection of the toner concentration. When the toner replenishment is carried out solely with the fixed timing under the condition involving quick consumption of the toner as described above, however, the replenishment of toner is made after the toner concentration is considerably lowered from the reference value and, as a result, the width of variation of the toner concentration in the developer is suffered to increase.